Approach to high temperature wafer processing

ABSTRACT

At temperatures near, and above, 385° C., gold can diffuse into silicon and into some contact materials. Gold, however, is an excellent material because it is corrosion resistant, electrically conductive, and highly reliable. Using an adhesion layer and removing gold from the contact area above and around a contact allows a Micro-Electro-Mechanical Systems device or semiconductor to be subjected to temperatures above 385° C. without risking gold diffusion. Removing the risk of gold diffusion allows further elevated temperature processing. Bonding a device substrate to a carrier substrate can be an elevated temperature process.

TECHNICAL FIELD

Embodiments relate to the fields of semiconductor processing, sensors,and Micro-Electro-Mechanical Systems (MEMS). Embodiments also relateproducing gold pads that can withstand high temperatures such as thoseencountered in bonding operations wherein a device substrate is bondedto a carrier substrate.

BACKGROUND

Microstructure products, such as pressure sensors, can be produced viasemiconductor wafer processing or MEMS processing, hereafter referred toas MEMS processing. The microstructure product can be fabricated on adevice substrate using current and well known MEMS processingtechniques. A further step that is often required is bonding the devicesubstrate to a handler substrate. Furthermore, gold pads or goldinterconnect schemes are often used because of gold's high reliabilityand low corrosion characteristics. Gold and silicon, however, have aeutectic melting point of about 377-385° C. and bonding operations oftenexceed that temperature.

Techniques known to those practiced in the art of wafer processing canbe used to fabricate a device on a device substrate. MEMS processing isa stepwise process with one step following another. One of the laststeps is contact formation.

FIG. 4, labeled as “prior art”, illustrates a device substrate 101 witha passivation layer 103 an a contact 102. The passivation layer 103 isoften a layer of silicon dioxide, often simply called “oxide”. Thepassivation layer 103 can also be silicon nitride, often simply called“nitride”. The contact 102 is an electrical contact, which is a part ofa microcircuit or MEMS device where electrical current or electricalvoltage is applied. FIG. 4 does not show a MEMS device element or anymicrocircuit element other than the contact 102. Those practiced in theart of MEMS are familiar with the processing steps leading up to andincluding contact formation. Furthermore, they are aware of the vastvariety of devices and circuits that are typically present on a devicesubstrate that has a contact 102. The contact 102 can consist ofplatinum silicide (PtSi) as an example of a typical contact material.

FIG. 5, labeled as “prior art”, illustrates a device substrate 101 witha contact 102, passivation layer 103, and an adhesion layer 104. Thestructure illustrated in FIG. 5 can be produced from that of FIG. 4 bydepositing a layer of material to form the adhesion layer. Thosepracticed in the art of MEMS know many ways to deposit a layer ofmaterial over a substrate. The adhesion layer 104 can consist oftitanium tungsten (TiW) or a similar material.

FIG. 6, labeled as “prior art”, illustrates a device substrate 101 witha contact 102, passivation layer 103, adhesion layer 104, and gold layer105. The structure illustrated in FIG. 6 can be produced from that ofFIG. 5 by depositing a layer of gold over the adhesion layer.

FIG. 7, labeled as “prior art”, illustrates a device substrate 101 witha contact 102, passivation layer 103, adhesion layer 104, and gold layer105 after patterning. The structure of FIG. 7 can be formed from that ofFIG. 6 by patterning the adhesion layer 104 and the gold layer 105. Thefact that the gold layer 105 does not completely cover the adhesionlayer 104 is a standard part of MEMS processing that is not special, butshould be noted.

Structures similar to that illustrated in FIG. 7 are produced on devicesubstrates in order to take advantage of the properties of the goldlayer. After patterning, however, the gold layer 103 remains over thecontact 102. Pinhole defects in the adhesion layer 104 offer anopportunity for gold to diffuse into the underlying contact. If apinhole or other defect in the adhesion layer 104 is present then thegold will be physically touching the underlying contact material 102.

The eutectic melting point of gold and silicon, approximately 385° C.,is the temperature at which gold that is touching the underlying siliconcontact will actually start to melt. If gold melts into this contact,then gold diffuses into the silicon device substrate, and a fabricationerror occurs resulting in a manufacturing failure. As mentioned above,an adhesion layer, also called a barrier layer, between the gold layerand the underlying substrate or contact is normally used to prevent thisfrom occurring, but these layers can have pinhole defects. Gold candiffuse through a pinhole defect into an underlying contact or into thesubstrate. Furthermore, this failure mode is avoided in more typicalwafer processing by making sure that the wafer never rises above thegold-silicon eutectic temperature (i.e. approximately 377-385 C)

In MEMS processing, a silicon (Si) wafer is commonly used as a devicesubstrate. A handler substrate is frequently required for mechanicalisolation or other reasons well known to those familiar with MEMSprocessing. This handler substrate is often a Si wafer or a glass wafer.A number of techniques can be used to bond the device substrate to thehandler substrate. Three of those techniques are anodic bonding, glassfrit bonding, and eutectic bonding.

In anodic bonding, the substrates can be bonded from approximately 300°C. up to nearly 500° C. by placing and clamping the substrates betweentwo metal electrodes. A high direct current (DC) potential is appliedbetween the electrodes creating an electrical field, which penetratesthe substrates. If the handler substrate is a glass that contains sodiumions then at the elevated temperature the sodium ions are displaced fromthe bonding surface of the glass by the applied electrical field. Thedepletion of sodium ions near the surface of the glass makes the surfacehighly reactive with the silicon surface of the device substrate. Thehigh reactivity results in a solid chemical bond between the twosubstrates.

In glass frit bonding; a viscous glass material is coated on one or bothof the wafers to be bonded. This frit is sometimes heat treated to driveoff solvents and binders. The wafers are then aligned if necessary andbrought together. The wafers are then clamped under pressure and heatedto temperatures that are typically in the range of 400 C to 550 C. Theglass frit flows and bonds to the two surfaces.

In eutectic bonding one substrate is coated with a first component of atwo component eutectic bonding system and the other substrate is coatedwith the second component. The substrates are heated and brought intocontact. Diffusion occurs at the interface and an alloy is formed. Theeutectic composition alloy at the interface has a lower melting pointthan the materials on either side of it, and hence the melting isrestricted to a thin layer. It is this melted eutectic layer that formsthe bond.

All of these wafer bonding techniques use temperatures that are abovethe gold-silicon eutectic, but are not so high that the fabricatedcircuit would be ruined or destroyed. FIG. 8, labeled as “prior art”illustrates a device substrate 101 and a handler substrate 801 that arebonded together by a bond 802.

Aspects of the embodiments directly address the shortcomings of theprior art by patterning the gold layer 105 such that it does not overlaythe contact 102.

BRIEF SUMMARY

It is therefore an aspect of the embodiments to select a devicesubstrate and to process the device substrate usingMicro-Electro-Mechanical Systems (MEMS) processing techniques up to thestep of forming a contact. More than one contact can be formed. Thecontact is made from an electrically conductive material such asplatinum silicide. Furthermore, contact formation often includesremoving an insulating layer, such as an oxide layer or nitride layer,where it overlies any area where a contact is desired.

It is a further aspect of the embodiments to deposit an adhesion layerand then a gold layer over the device substrate. A refractory metal suchas tungsten or tantalum can be used as an adhesion layer. Titaniumtungsten (TiW) is a well known and commonly used material that can alsobe used as an adhesion layer.

It is another aspect of the embodiments to pattern the gold layer andthe adhesion layer because both of those layers are electricallyconducting. Failure to pattern the layers would result in all thecontacts being electrically connected to one another. Furthermore, thegold layer is removed from the contact area so that subsequent hightemperatures do not result in the diffusion of gold into the contact.

It is an aspect of certain embodiments to bond the device substrate to ahandler substrate after the gold layer and the adhesion layer arepatterned. Many bonding process require temperatures above 385° C.Anodic bonding, glass frit bonding, and eutectic bonding are examples ofthe bonding processes that can be used to bond the two substratestogether. Properly patterning the gold layer in accordance with theembodiments disclosed here can ensure that gold doesn't diffuse into thecontact.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, in which like reference numerals refer toidentical or functionally-similar elements throughout the separate viewsand which are incorporated in and form a part of the specification,further illustrate the embodiments and, together with the detaileddescription, serve to explain the embodiments disclosed herein.

FIG. 1 illustrates a device substrate with a contact, passivation layer,adhesion layer, and gold layer after patterning;

FIG. 2 illustrates a top view of a device substrate with a contact area,patterned passivation layer, patterned adhesion layer, and patternedgold layer;

FIG. 3 illustrates a high level flow diagram for producing a devicesubstrate with a contact area, patterned passivation layer, patternedadhesion layer, and patterned gold layer;

FIG. 4, labeled as “prior art”, illustrates a device substrate with apassivation layer and a contact;

FIG. 5, labeled as “prior art”, illustrates a device substrate with acontact, passivation layer, and an adhesion layer;

FIG. 6, labeled as “prior art”, illustrates a device substrate with acontact, passivation layer, adhesion layer, and gold layer;

FIG. 7, labeled as “prior art”, illustrates a device substrate with acontact, passivation layer, adhesion layer, and gold layer afterpatterning; and

FIG. 8, labeled as “prior art” illustrates a device substrate and ahandler substrate that are bonded together by a bond.

DETAILED DESCRIPTION

The particular values and configurations discussed in these non-limitingexamples can be varied and are cited merely to illustrate at least oneembodiment and are not intended to limit the scope thereof.

FIG. 1 illustrates a device substrate 101 with a contact 102,passivation layer 103, adhesion layer 104, and gold layer 105 afterpatterning. Comparing the structures illustrated in FIGS. 1 and 6,notice that in FIG. 1 there is no gold above the contact 102. Oneskilled in the art of Micro-Electro-Mechanical Systems (MEMS) processingis familiar with the techniques for producing the FIG. 7 structure. Onconsideration of the aspects of the embodiments disclosed herein, oneskilled in the art of MEMS processing can use techniques to produce thestructure illustrated in FIG. 1.

FIG. 2 illustrates a top view of a device substrate 101 with a contactarea 201, patterned passivation layer 103, patterned adhesion layer 104,and patterned gold layer 105. FIG. 1 is a side view of a structure andFIG. 2 is a top view of the same structure. The contact area 201 isslightly larger than the contact 102 because there must not be any goldabove the contact. This is accomplished by the design of the photomasksused for patterning the metal layers 104 and 105 which is well known tothose skilled in the art. The manufacturing tolerances of MEMS processescause the contact area, which is devoid of gold, to be slightly largerthan the contact 102. Furthermore, there must not be gold close enoughto the contact that gold can diffuse through pinhole defects in theadhesion layer and into the contact. FIG. 2 shows the contact 102 eventhough it lies underneath the adhesion layer 104 because the adhesionlayer can be thin enough that underlying contact is visible.

FIG. 3 illustrates a high level flow diagram for producing a devicesubstrate 101 with a contact area 201, patterned passivation layer 103,patterned adhesion layer 104, and patterned gold layer 105. After thestart 301, a device substrate is selected 302 and processed up throughcontact formation 303. Next, an adhesion layer is deposited 304 and agold layer is deposited 305. The gold layer is patterned 306 followed bypatterning the adhesion layer 307. Patterning the gold layer includesremoving the gold from the contact area 201 after which the process isdone 308 or further processing can be accomplished in the typicalfashion. At this point, a device substrate that can withstand elevatedtemperatures without gold diffusing into the contact or substrate hasbeen produced. Further processing at elevated temperatures, such asbonding the device substrate to a handler substrate, can now be safelyperformed.

It will be appreciated by those familiar with semiconductor processesthat even though there is no gold above the contact area, there is stillgood electrical contact between the gold interconnect metal 105 and thecontact 102 (typically platinum silicide as mentioned earlier). Thisaccomplished through the adhesion metal layer 104 because the adhesionlayer is also an electrically conductive metal. In other words, the goldlayer 105 is electrically connected to the adhesion layer 104 which inturn is electrically connected to the contact 102.

It will be appreciated that variations of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A method comprising: producing a device substrate comprising asemiconductor or MEMS device electrically connected to a contact whereina passivation layer overlying the device substrate comprises a contactwindow over the contact; depositing an adhesion metal over the contactand the passivation layer to produce an adhesion layer and depositinggold over the adhesion layer to produce a gold layer; patterning thegold layer to remove gold from a contact area comprising the contactwindow; and patterning the adhesion layer such that the adhesion layerremains over the contact area; thereby producing a device substratecomprising the semiconductor or MEMS device and a gold interconnectionlayer that can withstand temperatures greater than 375° C.
 2. The methodof claim 1 wherein the contact material comprises platinum silicide. 3.The method of claim 1 wherein the adhesion layer comprises titaniumtungsten.
 4. A method comprising: producing a device substratecomprising a semiconductor or MEMS device electrically connected to acontact wherein a passivation layer overlying the device substratecomprises a contact window over the contact; selecting a handlersubstrate wherein the handler substrate is silicon or glass; depositingan adhesion metal over the contact and the passivation layer to producean adhesion layer and depositing gold over the adhesion layer to producea gold layer; patterning the gold layer to remove gold from a contactarea comprising the contact window; patterning the adhesion layer suchthat the adhesion layer remains over the contact area thereby producinga device substrate comprising the semiconductor or MEMS device and agold interconnection layer that can withstand temperatures greater than375° C.; and bonding the device substrate to the handler substrate,thereby producing at least one working semiconductor or MEMs device on adevice substrate bonded to a handler substrate.
 5. The method of claim 4wherein the contact material comprises platinum silicide.
 6. The methodof claim 5 wherein the adhesion layer comprises titanium tungsten. 7.The method of claim 6 wherein the bonding is anodic bonding.
 8. Themethod of claim 6 wherein the bonding is glass frit bonding.
 9. Themethod of claim 6 wherein the bonding is eutectic bonding.
 10. Themethod of claim 4 wherein the adhesion layer comprises titaniumtungsten.
 11. The method of claim 10 wherein the bonding is anodicbonding.
 12. The method of claim 10 wherein the bonding is glass fritbonding.
 13. The method of claim 10 wherein the bonding is eutecticbonding.
 14. A method comprising: selecting a device substrate andprocessing the device substrate up to the step of forming a contact froma contact material; selecting a handler substrate wherein the handlersubstrate is glass or silicon; depositing an adhesion metal over thecontact and other areas to produce an adhesion layer and depositing goldover the adhesion layer to produce a gold layer; a step for patterningthe gold layer to remove gold from a contact area comprising a contactwindow; a step for patterning the adhesion layer; and a step for bondingthe device substrate to the handler substrate; wherein gold does notdiffuse into the contact during the step for bonding the devicesubstrate to the handler substrate.
 15. The method of claim 14 whereinthe bonding is anodic bonding.
 16. The method of claim 14 wherein thebonding is glass frit bonding.
 17. The method of claim 14 wherein thebonding is eutectic bonding.